аЯрЁБс > ўџ E G ўџџџ D џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџьЅС 3 №П s% bjbj5G5G "T W- W- s! џџ џџ џџ l z z z z z z z  j j j j v $  Е І І І І І І І І 4 6 6 6 6 6 6 $ Е
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  z z z z й Fw: Bill ******* Voice Log Free comments on SBCs etc
John,
See below - perhaps I may finally get my points over - I expect that you have already covered belwo but now there is some evidence.
Kind regards
Bill ********----- Original Message -----From: "Bill *******" To: ******.Khan@****.com, Daniel.********@****.comSubject: Bill ******* Voice Log Free comments on SBCs etcDate: Thu, 28 Dec 2006 02:48:48 -0500
Director and Consultant
** Diagnostics Ltd
20 ******* Road,
Alford, Aberdeenshire, AB33 ***
Scotland, UK
Tel +44 19755 (0) *****
Fax +44 19755 (0) *****
http://www.*****motor.com
E-mail attachment follows :
Good morning ********* and Danny,
The time spent on a brief review of the results (see below) from the SBCs etc is NOT being logged against the contract, it is for my personal interest and to assist the investigative team, of course, I await a decision on whether you accept my proposal for additional work.
As already stated the vibration levels and vibration spectra on the motor when running uncoupled are perfectly acceptable - see my review document sent on 27th December, 2006 - maximum overall r.m.s. level = 0.89 mm/s on the motor bearing at the drive end - maximum allowable during FAT test = 2.3 mm/s and that is on a solid foundation.
I reiterate again, the vibration levels on the motor are perfectly normal, and in fact, are low compared to thousands of motors I have analysed.
As is normal, there are components at 29.99 Hz (call it 30 Hz) and harmonics of the 1xr.p.m. component,- the GT2 motor is 4 -pole and running uncoupled the speed will be very close to synchronous speed of 1800r.p.m. (30 Hz).
Under normal conditions, it is the 1x r.p.m. frequency component (30 Hz on no-load) due to inherent rotor unbalance that will dominate the vibration spectrum but there will normally be harmonics of the rotational speed component but at a smaller magnitude.
This is exactly what the GT2 motor is producing.
Consider the vibration spectra at the positions where the highest vibration levels were recorded on the motor and at the motor's feet when running uncoupled :
Motor, DE bearing Horizontal position : overall r.m.s. level 0.89 mm/s r.m.s. span 0 - 3000 Hz, 0.233Hz/line.
Spectrum : 30 Hz : 0.75 mm/s, 60 Hz : 0.34 mm/s, 90 Hz : 0.13mm/s, 120 Hz : 0.14 mm/s, 150 : 0.06mm/s, 180 Hz : 0.02mm/s.
Motor feet, NDE LH Horizontal : 0.77 mm/s r.m.s. 10 Hz - 140 Hz, 12,800 lines giving 0.01 Hz/line (no results for 10 - 3000 Hz were provided?)
Spectrum 29.99 Hz : 0.67 mm/s, 60 Hz : 0.16 mm/s, 89.96 Hz : 0.16mm/s, 120 : 0.06mm/s, 119.97 Hz : 0.02mm/s.
Levels at 150 and 180 Hz at the motor's feet are unknown since BV did not pass on the results between 10 - 3000 Hz recorded at the feet as per my spec.
The levels at the feet are perfectly acceptable ( low) when the motor is mounted on an offshore platform skid.
As we all know , but let us recall, the definition of mechanical resonance.
When the frequency of a forcing function applied to a structure equals a natural frequency of the structure then the structure is considered to be in resonance. Consequently the resulting vibration on the excited structure is a maximum at that frequency and the mechanical impedance (stiffness) is a minimum.
It is accepted that even if the magnitude of the forcing function is low the structure can still be resonating since it is the frequency which is the crucial parameter.
Clearly the inherent and normal vibration frequencies produced by the GT2 motor when running uncoupled will be always present.
Generalised statements that the SBCs are in resonance due to the motor (recent uncoupled test) need to be qualified since it suggests that the motor was the problem when the most recent measurements on the SBCs were taken.
The fact that the motor is producing vibration components that may be exciting natural frequencies of a number (??) of the SBCs (as reported by ****/BV) does NOT mean the motor has a problem/fault when operating uncoupled, on the contrary, it has been known since the beginning that the GT1 and GT2 motors can exhibit these normal vibration frequencies when running uncoupled, the associated structure/component parts should be designed to avoid resonant conditions.
The magnitudes of the frequency components produced by the motor (due to rotor unbalance and harmonics of) can be reduced by reducing the degree of rotor unbalance to a very minimum but since the levels are already low then to now remove the rotor for further balancing is NOT the way forward.
With respect to the levels on the SBCs with the motor running uncoupled, the focus has to be on the response of the SBCs.
It is accepted that vibration results have still to be taken with only the cooling fans running and then a further review can be carried out.
EM Diagnostics was not charged with reviewing the SBC results/natural frequencies but let us consider :
Results produced by BV on measured natural frequencies dated 6/7th December.
Consider PZA 2235 HP suction line, the measured natural frequencies along the line and across the line, both sated in the table to be 175.5 Hz, this result certainly suggest that there are no other dominant natural frequencies. The value of 175.5 Hz does not coincide with any of the forcing function vibration components from the motor, the closet from the motor being at 180 Hz but at very low level of 0.02 mm/s (very close to the noise floor) and of course, the 175.5 Hz is sufficiently removed from the 180 Hz to avoid resonance (or keep its influence to a very minimum.
However, the vibration level recorded by BV on PZA 2235 HP during the recent uncoupled run was 7.22 mm/s and 5.098 mm/s in the axial and radial positions respectively and the frequency was stated to be at 30 Hz in both positions.
The fact that the natural frequency of the SBC was at 175.5 Hz, BUT yet the vibration level measured was at 30 Hz suggests an anomaly. If the SBC was in resonance then it would have been expected that its natural frequency would have been at 30 Hz (or close to) to give the level of 7.22mm/s but the natural frequency was reported to be at 175.5 Hz!
Consider PG 498 HP Suction, the measured natural frequencies along the line and across the line, stated in the table to be 30 and 29.5 Hz, respectively.
In this case it would be very reasonable to propose that if PG 498 is being forced into resonance by the 30 Hz component from the motor and that the level on this SBC would have been high BUT that was not the case, the levels when the motor was running uncoupled were 2.25 mm/s and 1.49 mm/s in the axial and radial positions which is perfectly acceptable (3 mm/s set by **** as the upper level on the SBCs).
There are other anomalies in the results presented to EMD but of course I may not have the full picture with respect to the SBCs that have been stiffened and those that have not in drive train GT2 etc.
However, what is clear, is that a complete picture and review of the natural frequencies measured on the SBCs (before and after stiffening), positions of accelerometers, differences between SBCs, resolutions of measurements, background levels on the SBCs with only GT1 running etc is required.
There are anomalies between reported natural frequencies and what was actually measured on for example, 2 of the SBCs (as above) when the motor was running uncoupled.
Also, the SBC PG 2228 IP discharge had a level of 10.8 mm/s when the motor was running uncoupled but I cannot find the natural frequencies in the table etc passed to me? Clearly the natural response of that SBC has to be measured, and the background vibration recorded for cross correlation with the 10.8mm/s at 30 Hz (as per BV's results).
My fundamental question is what are the natural frequencies measured on the SBCs actually representing - what is truly the source of the natural frequencies being measured on the SBCs? There is a wide variability in the natural frequencies presented by BV (definitive reasons should be given with respect to structural differences, positions of accelerometers, nodes and anti-nodes etc). Note BV's results as measured are not being questioned but the fundamental interpretation needs to be justified.
I now await feedback on my proposal for additional work.
Kind Regards
Bill ********
Director and Consultant ** Diagnostics Ltd 20 ****** Road, Alford, Aberdeenshire, AB33 *** Scotland, UK Tel +44 19755 (0) ****** Fax +44 19755 (0) ****** http://www.*****motor.com
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